(1) Field of the Invention
The present invention relates to a high pressure discharge lamp that is used in general lighting fixtures and optical instruments, and also relates to a high pressure discharge lamp electrode, a method of producing the high pressure discharge lamp electrode, and an illumination device and an image display apparatus respectively using the high pressure discharge lamps.
(2) Description of Related Art
Conventionally, a light source and a concave reflecting mirror are usually formed in one piece as an illumination device that is provided in an image display apparatus, such as a liquid crystal projector. As a light source of the illumination device, a high pressure mercury lamp with a short arc, which is close to a point light source, has been used. The high pressure mercury lamp has advantages, such as an excellent luminous efficiency, high intensity, favorable balance of red, blue, and green in emitted light, and long life. As one example of a high pressure discharge lamp, a conventional high pressure mercury lamp is described below.
In general, a high pressure mercury lamp is provided with a discharge tube having a light-emitting part and a pair of sealing parts. The light-emitting part includes a pair of electrodes. The light-emitting part is filled with mercury as light-emitting material, a rare gas such as argon gas for starting-up, and halogen substance that facilitates a halogen cycle during lamp operation.
FIG. 1 shows an example of an electrode that has been used in this conventional high pressure mercury lamp. As shown in this figure, a conventional electrode 901 is composed of a coil 903 and an electrode rod 902 both made of tungsten, with the coil 903 being set at a discharge side end of the electrode rod 902. The coil 903 has a closely-wound double-layered structure. Specifically, a first layer 903a has 15 turns while a second layer 903b consisting of 8 turns is wound around the first layer 903a. 
When this high pressure mercury lamp is lit up, a temperature of the end of the electrode considerably increases. As a result of this high temperature of the electrode end, even though halogen substance has been inserted in the light-emitting part, tungsten used for making the electrode is deposited on an inner surface of the discharge tube, causing blackening. This gives rise to a problem that a lamp life is shortened.
The techniques for preventing blackening from occurring to the discharge tube are disclosed in U.S. Pat. No. 5,357,167 and Japanese Laid-Open Patent Application No. 10-92377.
FIG. 2 shows an electrode disclosed in U.S. Pat. No. 5,357,167. As shown in this figure, an electrode 911 is composed of an electrode rod 912, a sleeve 913, and an electrode end 914. The electrode rod 912 and the sleeve 913 are both made of refractory metal, such as tungsten and molybdenum. The sleeve 913 is positioned on the electrode rod 912. The hemisphere-shaped electrode end 914 is formed by melting the metals respectively forming the electrode 912 and the sleeve 913 by heat, thereby being integrally joined to both the electrode rod 912 and the sleeve 913. With the construction disclosed in this reference, a heat capacity of the end of the electrode is increased. Therefore, blackening caused by the deposition of refractory metal, such as tungsten, is prevented by suppressing overheating of the end of the electrode. Also, the heat flow of the electrode rod 912 is controlled owing to the small diameter of the electrode rod 912, so that the temperature of the electrode end 914 can be prevented from falling below the temperature required for discharge.
Meanwhile, Japanese Laid-Open Patent Application No. 10-92377 discloses an electrode (referred to as the xe2x80x9celectrode 921xe2x80x9d) as shown in FIG. 3 and a method of producing the same. More specifically, the electrode 921 includes an electrode rod 922 that is made of tungsten and partially covered with a covering material 923. Here, the discharge side tip of the electrode rod 922 is left uncovered. With this state, a discharge takes place between the end of the electrode rod 922 and a discharge electrode (not shown in FIG. 3) under an inert gas atmosphere. As a result of this discharge, the tip of the electrode rod 922 that was left uncovered is melted. Then, the melted part that has solidified in the shape of a rough sphere or a pear is shaped by polishing or grinding, so that an electrode end 924 is formed. In this way, the electrode 921 shown in FIG. 3 has been produced.
However, after an acute analysis, the inventor of the present invention found that various problems could arise if actually producing electrodes using the methods disclosed in the cited references. The inventor further conducted an analysis, and then came up with the present invention that addresses the various problems. The problems found through the analysis by the inventor and details how the inventor came up with the present invention are explained below.
The inventor first employed the method where an electrode rod is covered with a sleeve or coil and the end of the electrode rod is melted, as disclosed in the cited references. As a result, the shape of the solidified end of the electrode rod was unstable in most cases and had to be machined to form an appropriate shape through such as polishing and grinding. Additionally, the inventor found that blackening could not be adequately prevented in an actual use.
To be more specific, the inventor had the end of the electrode rod melted, with the tip of the rod being left uncovered with the sleeve or coil serving as the covering material. As a result, the shape of the solidified end of the electrode rod was not suitable for the actual use. In most cases, the solidified end needed to be machined to be formed into an appropriate shape through such as polishing or grinding as described in Japanese Laid-Open Patent Application No. 10-92377.
Meanwhile, the inventor conducted another experiment where the melting process was carried out, with the coil that covered the electrode rod being extended comparatively longer to the discharge side than the end of the electrode rod. In this case, the inventor found that there might be a case where blackening could not adequately be prevented. The inventor examined the electrode that had been produced in this way and found that there was a void appearing between the coil and the electrode rod. Here, it was the coil that was mainly melted, and the electrode rod remained as it had been without being melted. As can be understood, a void reduces the heat capacity of the electrode end. This leads to overheating of the electrode end in the actual use, meaning that blackening caused by the deposition of tungsten cannot be prevented.
In accordance with these findings, the present invention addresses the stated problems. The object of the present invention is to provide a high pressure discharge lamp that can prevent blackening, a high pressure discharge lamp electrode whose end does not need to be machined after melting, a method of producing the high pressure discharge lamp electrode, and an illumination device and an image display apparatus respectively using the high pressure discharge lamps.
The object of the present invention can be achieved by a high pressure discharge lamp made up of: a discharge tube having a discharge chamber that contains a light-emitting substance and is hermetically sealed; and a pair of electrodes, each of which has first and second ends and is set in the discharge chamber, the first end of each electrode being secured to the discharge tube and the second ends of the electrodes facing each other at a predetermined distance in the discharge chamber, wherein discharge takes place between the second ends of the electrodes, each electrode made up of an electrode rod with a tip and a covering material, the electrode rod and the covering material being made mainly of tungsten and the tip positionally corresponding to the second end, wherein the covering material covers an outer surface of the electrode rod near the tip, the tip being left uncovered, and the tip of the electrode rod and an adjacent portion of the covering material are fused together by heat generated during an initial discharge, and wherein an inequality 1/50*R3xe2x89xa6xcex94Lxe2x89xa61/5*R3 is satisfied before the initial discharge takes place, where xcex94L is a length of the tip measured along a direction of a length of the electrode rod and R3 is an outer diameter of the covering material adjacent to the tip.
For this high pressure discharge lamp, the end of the electrode is melted by heat when an initial discharge takes place between the electrodes, so that the electrode rod and the coil are integrally joined to each other at the end of the electrode. It should be noted here that an arc length between the electrodes may vary in a case where the electrodes are set in the discharge tube first and then the electrode ends are melted by heat. However, it became apparent from the analysis by the inventor that the problem associated with the changes in the arc length would be solved when the following Inequality (1) is satisfied.
1/50*R3xe2x89xa6xcex94Lxe2x89xa61/5*R3xe2x80x83xe2x80x83(1)
In Inequality (1), R3 indicates an outer diameter (mm) of the discharge side end of the covering material while xcex94L indicates a length (mm) of the discharge side end of the electrode that is left uncovered with the covering material such as a coil.
In accordance with this finding, the inventor came up with an invention of a high pressure discharge lamp electrode that can solve the stated problems of the prior art. With the construction that satisfies Inequality (1), the arc length will not vary after the end of the electrode has been melted for forming the integral joint. Specifically, this construction can avoid a case where the shape of the electrode end becomes unstable after the melting by heat, i.e. a case where the arc length is increased since it is the electrode rod that mainly melts. Also, this construction can avoid a case where a void appears between the covering material and the electrode rod, i.e. a case where the arc length is reduced since it is the coil that mainly melts and the molten coil bulges due to the void.
The stated problems of the prior art can be solved by a high pressure discharge lamp electrode made up of: an electrode rod which has a tip and is made of a refractory metal; and a coil which is made of a refractory metal wire and covers an outer surface of the electrode rod near the tip, a portion of the coil adjacent to the tip being melted so as to be fused in tight contact with the tip which does not substantially melt and remains in an initial shape.
To be more specific, the electrode end does not need to be machined through such as polishing or grinding after the integral joint. Moreover, blackening caused by overheating of the electrode end can be prevented from occurring to the discharge tube.